1. Field
Embodiments of the present invention relates to a lithographic apparatus and a method for manufacturing a device.
2. Background
A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning” direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
Overlay performance, i.e., the accuracy of positioning one layer of the IC over another layer of the IC, is known to be affected by exposure light induced reticle heating. The absorbed energy from the radiation beam during exposure heats up the reticle, causing the reticle to expand. This expansion leads to a time and position dependent pattern displacement on the wafer. The size of the effect is influenced by the used exposure energy, reticle transmission, exposure field size, and expose time.
In some lithography apparatus, a sensor is used to measure the actual reticle expansion over time. The measurement results are used for correction purposes by control software during wafer exposures.
The lithographic apparatus and known methods can cope with a uniform heated reticle, referring to the situation wherein reticle marks are close to the exposed reticle area, the reticle marks being used for reticle alignment relative to the pattern already created on the substrate. In case of uniform reticle heating, the arrangement of the reticle marks close to the exposed reticle area allows sufficient correction. However, it is observed that if only a part of the reticle is exposed, the reticle may expand in a non-uniform way which results in local (thermal) deformation of the reticle pattern, in contrast to the case of uniform heating which results basically in an overall thermal expansion. Due to the non-uniform expansion, the known methods do not provide a sufficient correction.